c. 1600, "rope used in towing," from tow (v.). Meaning "act or fact of being towed" is from 1620s.

tow (v.)

"pull with a rope," Old English togian "to drag, pull," from Proto-Germanic *tugojanan (source also of Old English teon "to draw," Old Frisian togia "to pull about," Old Norse toga, Old High German zogon, German ziehen "to draw, pull, drag"), from PIE root *deuk- "to lead" (source also of Latin ducere "to lead"). Related: Towed; towing.